The present invention relates generally to LED power supplies. More particularly, the present invention relates to an LED driver having transient protection circuitry designed to satisfy dielectric testing requirements.
A conventional LED power supply 10, an example of which is represented in FIG. 1, may typically include a line fuse 12 and electromagnetic interference (hereinafter “EMI”) filtering circuitry 13 provided between a sinusoidal AC source 11 and an input rectifier 14. The rectifier and a subsequent power factor correction (hereinafter “PFC”) stage 15 convert an AC input to a high voltage DC output, further regulating the power factor and total harmonic distortion. A DC-DC converter stage 16 receives the high voltage DC output from the PFC stage, and is further configured to regulate the output voltage and current to a load 17.
Energy storage elements are conventionally used in DC-DC power converters for filtering, or “smoothing,” pulsating DC input from a rectifier or PFC stage by absorbing peak currents and ripple currents while providing a relatively constant DC voltage output. For a 120-277V input driver, the output voltage of the PFC stage is typically around 470V DC. For a 347V input driver, the output voltage of the PFC stage is around 600V. Because of the relatively high voltages involved, first and second electrolytic capacitors C1, C2 are often provided as energy storage elements at the output of the PFC stage. In the particular case of a 470V output, two 250V electrolytic capacitors may be used, whereas for the 600V output a pair of 350V electrolytic capacitors may be implemented. These electrolytic capacitors are connected in series to satisfy the voltage rating requirement.
Such conventional configurations are potentially susceptible to failures that may occur where one of the electrolytic capacitors is shorted.
In one particular and contemporary example, all LED power supplies must be designed to pass an abnormal component fault test as administered according to the UL 8750 standard. One part of this abnormal component fault test involves shorting one of the output electrolytic capacitors, after which the respective power supply must pass the UL dielectric test (i.e., a leakage current test between power source and earth ground).
However, when one of the two electrolytic capacitors is shorted, all of the DC output voltage from the PFC stage will be applied across a single electrolytic capacitor. As a result, the voltage rating of the single electrolytic capacitor would be greatly exceeded, assuming a practical voltage rating as previously noted. Further assuming that the increased voltage across the remaining electrolytic capacitor does not exceed a rating for the fuse 12, the fuse will not open or otherwise prevent subsequent electrolytic capacitor failure, which in turn causes the electrolytic capacitor to blow up and/or substantially discharge the liquid electrolyte included therein. The electrolyte is conductive and may subsequently short the circuit on an associated PCB, which could further form a short circuit to earth ground or the device enclosure. The power supply would accordingly fail the UL leakage test due to this short circuit, as caused by the exploded electrolytic capacitor.
It would therefore be desirable to provide an LED driver with circuitry that prevents total electrolytic capacitor failure in the event that a short condition is sensed in a corresponding electrolytic capacitor.